1. Field of the Invention
The invention relates generally to the field of photography, and, in particular, to reducing the likelihood of so-called film creep, i.e. the movement of the camera film during exposure, which can occur if a camera element that prevents advancement of the film is deactivated either immediately before of during the opening of the camera shutter.
2. Description of the Prior Art
Many cameras employ a film advancing mechanism comprising a toothed metering wheel that is coupled to a rotatable film take-up core or spool and a metering mechanism comprising a metering member including a pawl that engages the metering wheel to stop the film advancement each time an unexposed image area of the film is brought into exposure position. In a typical 35 mm camera, the metering member is spring biased so that a tooth thereof bears against the edge of a notched actuator disk driven by an eight-tooth sprocket that is engaged with the metering perforations along an edge of the film; each exposure frame of the film corresponding to eight perforations. When the sprocket has rotated the actuator disk through a complete rotation, i.e. when the film has been advanced by one frame, the tooth of the metering member falls into the notch of the actuator disk. This activates the film metering member by allowing it to move to engage the metering pawl with the metering wheel, thereby temporarily blocking film advancement.
Following each exposure, the above-described mechanism must be deactivated in order to allow the film to be advanced to the next available film frame. In other words, the metering pawl must be disengaged from the metering wheel and the metering lever tooth must be withdrawn from the notch in the actuator disk.
In many relatively inexpensive cameras, the shutter is operated by a spring-loaded driver member, often referred to as a high energy lever. The driver member is cocked by the film advancing mechanism and is latched in its cocked condition until it is released by depression of the shutter release button. Thereupon, the driver member moves rapidly to an uncocked position, and, in so doing, momentarily opens the shutter to make an exposure. Additionally, various prior art cameras of this type use the shutter driver member to disable the film metering mechanism as the driver member approaches its uncocked position; thereby allowing the subsequent film advancing operation to be performed.
With such a mechanism, it is very likely that the film will be under a certain amount of tension when the metering wheel is locked by the metering pawl and the rotation of the sprocket is limited by the tooth in the notch of the actuator disk; or, stated another way, it is likely that the film will exert a certain amount of rotational force on the film winding core and/or on the sprocket. Furthermore, the metering wheel is also typically engaged by another resiliently biased pawl that prevents it from rotating in the opposite direction; and the interaction between this pawl and the metering pawl may also impart a rotational force to the metering wheel. Accordingly, when the metering mechanism is deactivated, the winding core and the sprocket are now free to rotate and to thereby relieve whatever forces were latent in the pawl arrangement or in the tensioned portion of the film; thereby tending to cause the exposure portion of the film slightly to move slightly relative to the axis of the camera's lens.
If the foregoing film movement occurs while the shutter is open, as is very likely if the deactivating operation is effected by the shutter driver member or high energy lever, as described above, the photograph will be blurred to a degree determined by the severity of such film movement or creep.
In an alternative type of metering mechanism, used, for example in various cameras designed for 110 or 126 size film, the metering pawl is engaged with the metering wheel when a film sensing tooth enters into a metering perforation in the film or is moved sideways by the film after having entered such a perforation. With such a mechanism, the deactivating operation includes retraction of the sensing tooth from the film rather than the removal of the metering member tooth from the actuator disk, but the problem of film creep is otherwise the same as described above.
To avoid or minimize the foregoing film creep problem, the invention to which the above-identified U.S. Patent application Ser. No. 08/313,818 is directed contemplates providing the shutter release mechanism with a brake member that engages the metering wheel as the shutter release button is depressed but before the metering mechanism is deactivated, thereby braking the metering wheel against rotation as long as the button remains depressed; i.e. during the deactivation of the metering mechanism. This approach, however, dictates the need for rather demanding tolerances to insure that the brake is engaged before such deactivation takes place, while at the same time avoiding excessive travel of the release button. Also, it requires that the release button be depressed with sufficient force to insure positive braking of the metering wheel. Furthermore, such an arrangement does nothing to avoid film creep that may be occasioned by the disengagement of the metering member from the film metering sprocket or its equivalent.
To address tile above-described problems inherent in the type of device to which the foregoing '818 Patent Application is directed, the invention to which the later '807 Patent Application is directed contemplates a camera of the previously described type in which the metering mechanism is deactivated by a shutter driver or high energy lever as the latter approaches its uncocked condition; but in which the shutter release device includes a blocking member that engages the activated metering member when the shutter release button is depressed and thereby temporarily prevents the metering member from being deactivated. When the button is allowed to return to its raised position, the blocking member releases the metering member so that the latter can be moved to its deactivated position by the final movement of the shutter driver to its uncocked position. However, by this time, the exposure has been completed and any film movement occasioned by the deactivation of the metering mechanism cannot blur the photograph. This construction, therefore, affords a very significant improvement. However, its successful operation is still dependent on satisfying rather exacting dimensional tolerances with regard to the relative positions of components of the shutter release and film metering mechanism. This is particularly critical if the periphery of the metering wheel is provided with relatively fine teeth, which is desirable to increase the accuracy of the metering mechanism.